OK, hmm, thinking...
Music. It's one thing that can really help, because tunes are repetitive and reliable and predictable. I had the good fortune to be given piano lessons when very young and a family who rescued an ancient wreck of a piano so I could practise my playing. It was an escape for me when life was too stressful, and even now I use piano as a way to calm down. Same with music tracks - if stressed I'll play the same track 10, 20 times over if I know it won't drive the rest of the family to distraction. Kid's Headphones and a robust music player might be an answer, for example.
Trying to think how to explain what happens in our brains (assuming he's ASD, which is a big assumption but let's just assume it for a minute anyway). In brains, there's a bit called the amygdala. It recognises emergencies and anything unexpected and angry faces. When it sees any emergency, it hits a big red Panic Button in the brain. OK, not a real one, but you know what I mean.
Now in an ordinary brain, immediately the brain's filing clerk appears, takes the info about what's going on, whizzes over to the filing cabinets in the brain to find out what it might be like/who it is that's angry, and whizzes back again to say "hey, it's ok, no emergency here - switch the Panic Button off again". Only takes a couple of seconds as most people have brilliant people-and-emergency filing systems. (Ok, there isn't really a filing clerk either, but there's a bit of the brain that does exactly that).
In our brains, the info on the emergency/something unexpected comes in, the amygdala bit of the brain presses the Big Red Panic Button, and, umm, er, no filing clerk. We don't have one. Imagine what sort of mess the filing's in, too. "Red Alert! Emergency! Er, excuse me, anyone there? Anyone got a clue why this has happened?" Nothing. So...we have to go look, painstakingly sorting through all the paperwork scattered all over the floor. It takes AGES and is very exhausting, and meantime we've no clue how dangerous the situation is, because we can't find the bloomin papers that tell us if it's a tiny thing or a huge great thing.
Result - loads of fear, loads of crying.
What helps - a) knowing this b) keeping things as predictable as possible c) helping us to work it out. This is where the visual things are handy, e.g. if it's a tiny emergency, holding up just one finger. If it's a big one, holding up all five fingers. Also, minimising all the incoming info - no talking to us, no eye contact, no extra noise, no changes...just let us calm down. A heavy blanket or coat on us can help a lot, or a thick duvet to wrap round ourselves. The rocking is a way to block it out, too.
Then wait. And know you're not a bad parent and didn't cause this.